Can I use my NEXUS Card instead of a passport?

If you are travelling internationally, you cannot use your NEXUS Card in place of your passport.
If you are travelling between the US and Canada, your NEXUS Card is accepted as proof of your citizenship when returning to the country of citizenship listed on your NEXUS Card by car, but is not normally accepted for air travel or for entering the country you are not a citizen of. (This is at the discretion of the office, though.)
If you are travelling domestically, you should be able to use your NEXUS Card as your photo ID.